Screw extruders have the disadvantage of frequently obtaining poorer qualitative results when homogenizing, kneading and dispersing solids in a viscous matrix or homogenizing substances of very different viscosity than with batchwise kneading. This also applies to twin screw extruders. To improve homogenization, the two shafts of twin screw extruders are therefore generally provided not only with screw elements but also with rotationally fast so-called kneading blocks which consist of two or more disk cams mutually offset at an angle in the circumferential direction and with a contour formed of a plurality of circular arcs. Such kneading blocks are described for example in DE 813 154 B and EP 0 422 272 B1.
Further, EP 0 788 867 B1 discloses an apparatus for continuous processing of flowable materials. In this apparatus, homogenization is also effected substantially by kneading blocks. Kneading blocks have the disadvantage, however, that low energies are conducted into the product to be processed in the deep channel center while very high energies are conducted in the channel tapering in a wedge shape to the housing, which can lead not only to a specifically high energy consumption but also to partial overstressing of the product and high wear in the area of the kneading block and housing. Moreover, the axially open construction of the kneading blocks can cause product fractions to pass through the kneading blocks easily on a short path and without energy consumption, i.e. without homogenization.
DE 28 54 207 B2 discloses a screw extruder having a mixing zone with a length of 4–12 D (D=outside diameter of a screw element), the pitches being between 0.5 to 2 D in the feed zone and 0.3–1 D in the discharge zone.
The problem of the invention is to state the requirements for the design of a generic apparatus for perfect homogenization and/or dispersion.